The first airbag systems for automobiles were developed in the 1970's. Since then, airbag systems have saved lives and prevented or reduced serious injury in numerous automobile crashes. Statistically, the effectiveness of airbag systems is without question. The success of airbag systems has also prompted their use in areas other than automobiles. In recent years, airbag systems have been developed for helicopters and general aviation aircraft. Airbags are also being used in various recovery systems, as energy absorbing devices, to reduce the landing impact of aircraft escape capsules, rockets or other space vehicles, and to reduce the landing impact of military cargo drops. Despite several years of development, improvement, and widespread use of airbag systems, problems still remain.
Where airbags are used for vehicle recovery or for cargo drops, problems are primarily related to poor efficiency, and therefore to excessive bag height which can result in payload rollover. In such uses, airbag performance requirements are generally described by the maximum impact force permitted (deceleration) and the mass and velocity of the payload at touchdown. Maximum efficiency is achieved when the system operates at a constant deceleration force slightly less than the maximum permissible deceleration force. This results in the minimum possible distance over which the kinetic energy of the payload can be absorbed.
When airbags are used for vehicle occupant protection, system efficiency is also very important. Of greater concern however, are system performance, reliability and safety considerations. Although a statistically small number, there have been some incidents where the airbag caused severe injury or even death. Many of these incidents have occurred in what is commonly called an out of position situation (OOPS). Simply stated, the occupant is too close to the airbag when the airbag deploys.
Some of the airbag induced injuries are due to crash sensor systems which do not adequately discriminate between crashes and minor impacts.
Some injuries are due to the very aggressive airbag developed in the United States because of requirements for protecting occupants not wearing lap and shoulder belts. The less aggressive airbags developed in Europe, where unbelted occupants are not a design concern, inflict fewer injuries. However, even with perfect sensors and less aggressive airbags, some out of position occupants would still be injured.
Some other airbag induced injuries relate to the wide variation in occupant size and weight. Conventional airbag systems are designed to produce a fixed set of performance parameters, e.g. inflation time, initial pressure, and venting. This set of parameters is intended to protect the widest possible range of occupant sizes. Unfortunately, the system may not provide adequate protection for a very large occupant and conversely, may be injurious to a very small individual.
These cases of airbag injury have attracted considerable media attention, especially when children are involved. This negative publicity has somewhat overshadowed the benefits of airbags, and has caused a fear of airbags among some vehicle owners. Some are even opting to have a lockout switch installed so the airbag system can be completely turned off. Doing so will indeed prevent airbag induced injuries but, unfortunately, the vehicle occupants are also forfeiting any possible benefits of the airbag system.
A unique problem also exists in the present U.S. Army cockpit airbag system (CABS) for Blackhawk, Seahawk, and Kiowa helicopters. These airbag systems are not vented like auto airbag systems are vented. The reason is that the typical crash scenario is much more protracted (e.g. tree strikes prior to ground impact or effects of very rough terrain) so a longer period of bag inflation is required. Therefore, the design and production of the inflator must be very precise to achieve the proper initial pressure. This is particularly difficult to achieve under the temperature extremes in which these helicopters operate. In very cold temperatures, the inflator must provide a certain minimum bag pressure for crew member protection. Unfortunately, in some instances, similar inflators may cause bag ruptures during high temperature use.
Another problem with conventional airbag systems is their size and bulk. This is particularly true of passenger airbag modules. Typical airbags must be larger than their ideal size because of their relatively inefficient fixed vent design. The "oversize" bags then require bulky modules for stowage and increase chances for airbag induce injury.
An ideal airbag system would inflate to a pre-determined pressure, provide an acceptable level of deceleration for the occupant, and maintain that deceleration at a nearly constant value during a crash event. The system would be adjustable to provide the proper deceleration for various size occupants. It would also have the ability to prevent serious injury to any occupant, by venting a large amount of propellant gases very early in the inflation cycle if the occupant is too close to the airbag. In contrast, a typical automotive airbag module only has nonadjustable vents in the airbag fabric. This conventional approach of "one size fits all", presents obvious compromises relative to occupant size and crash situation. Also, having vents in the airbag fabric requires that the airbag must unfold before any gas flow can reach the vents. In a very close OOPS, all of the inflation gases are confined in the airbag module creating a very high pressure, and therefore, a potentially hazardous force on the occupant.
The high media publicity focused on these problems (especially those in the public domain) has prompted numerous proposed solutions. Many of these proposed solutions address a "depowered" airbag, which will deploy with less velocity. This approach can reduce the incidence and severity of airbag induced injuries in minor crashes, but may also compromise the performance of the airbag system in severe crashes.
In proper system operation, the airbag inflates before the occupant enters the area that will be occupied by the airbag. A design rule of thumb, that has appeared in the literature over the years, is that the airbag must be fully deployed before the occupant has moved forward (due to crash acceleration) more than 5 inches from the normal sitting position. Some crash sensors perform this calculation and do not fire the inflator if the criterion is not met. While this prevents possible airbag induced injury, it follows that any benefit that might have been provided by the airbag has also been defeated.
Other proposals include a great variety of sensors intended to detect the size and position of seat occupants (especially the passenger) and microprocessor circuitry programmed with appropriate logic to control airbag deployment. Depending on the specific crash situation, these "smart airbag systems" may deploy using the full power of dual inflators, deploy with less force by using only one inflator, or not deploy at all. Again, if the system does not deploy, any possible benefit during a crash event has been forfeited.
Considerable research on improving the efficiency of cargo drop airbag systems has been conducted or sponsored by U.S. Army Soldier Systems Command, Natick, Mass. Numerous studies have been conducted with airbags having fixed exhaust vents. Studies have been conducted with various auxiliary devices. One such system involved injecting compressed air into an airbag while the airbag was being compressed. Another system, described in ASME Paper No. 091-WA-DE-1, uses a servo-controlled, mechanical sliding vent closure to affect greater system efficiency. A recent research program conducted by Warrick and Associates Inc. (ref. U.S. Army Soldier Systems Command, Natick, Mass., Contract No. DAAK-97-C-9204) has also demonstrated the efficiency advantages of maintaining a constant "ride-down" pressure in a cargo-drop airbag system. That system utilizes a pneumatic pilot-pressure feed back loop with flexible diaphragm valving. Although the size and complexity of such systems are not appropriate for personnel protection in passenger vehicles, the concept of using venting control to improve airbag efficiency has been clearly validated.
SAE Technical Paper Series Number 980646, "An Innovative Approach to Adaptive Airbag Modules" by Ryan, describes a valve developed to control the gas going into the airbag rather than controlling the gas exiting the airbag. Depending upon crash severity determinations made by the crash sensor, some gas may be diverted at the time of airbag inflation.
U.S. Pat. No. 5,219,179 to Eyrainer describes airbag valves which are essentially burst discs. These valves simply open at a pressure which is selected at the time of design. After opening, these valves function as fixed vents much the same as conventional airbags.
U.S. Pat. No. 5,310,215 to Walner shows conventional fixed vents overlying deflectors to minimize injury to the occupant. There is no provision for maintaining constant pressure.
U.S. Pat. No. 5,489,117 to Huber shows reed valves designed to operate at a very low pressure, and these valves are used to allow aspiration of ambient air during the inflation process. Although vent control is disclosed, the vent valves are designed to provide only two levels of fixed vent area and have no provision for maintaining a constant pressure.
U.S. Pat. No. 5,505,485 to Breed shows a spring-biased cover as " . . . vent means . . . for deflating said airbag". There is no mention of the cover's purpose being other than a means of quickly venting the "excess" gases. There is no specific mention of controlled venting, and indeed, it seems obvious that the cover could not serve such a purpose. The spring tabs shown would have a spring constant much too high. It appears that the cover simply remains closed, until the selected pressure is reached, and then, swings open, bending the "spring" tabs with it. Also of importance is the fixed nature of the cover. The cover is not adjustable in any way to vary the pressure for different occupant sizes.
U.S. Pat. No. 5,538,279 to Link et al shows a fixed vent (or vents) initially closed by a cover flap. The text repeatedly states that the cover will only open the vent port(s) after a pre-determined pressure is reached, but there is no attempt to explain how that occurs. It appears that the cover flap does little more than aerially distribute and re-direct the exhaust gases.
U.S. Pat. No. 5,603,526 to Buchanan shows fixed vents in the bag fabric, which are initially closed by frangible coverings. Functionally, this is very similar to the Eyrainer patent, previously referenced, and is apparently unique only in detail construction.
U.S. Pat. No. 5,695,214 to Faigle et al shows various methods of pre-selecting different fixed vent openings. Several devices are shown, including hinged doors, deformable doors, and explosive rivets or bolt releases. In all cases, once a vent-area setting has been selected, the vent area remains constant throughout system operation regardless of pressure.
U.S. Pat. No. 5,707,078 to Swanberg et al shows a mechanical valving system that pre-selects exhaust vent area, and simultaneously selects flow area from the inflator into the bag. As with the Faigle patent above, once the vent area is selected, the vent area remains constant throughout system operation.
U.S. Pat. No. 5,709,405 to Saderholm et al shows another mechanical means of pre-selecting flow area to control mass flow into the bag.
U.S. Pat. No. 5,853,192 to Silkorski et al shows yet another means of pre-selecting vent area with hinged doors and latches.
Although their purposes are stated somewhat differently, these last four patents, to Faigle, Swanberg, Saderholm and Silkorski, all do essentially the same thing. Their pre-set vents act as proportioning devices, wherein a portion of the inflation gases is directed toward the airbag while the remainder is directed to atmosphere. In all of these cases, where vent area is pre-selected as a result of various sensors, the areas selected are based on a presumed or anticipated inflator output. Even if it were possible to perfectly measure the critical variables and correctly discriminate the crash conditions, system performance would be vulnerable to inflator variations because no means of actual pressure control is provided. Elimination of inflator-specific variations is virtually impossible because of manufacturing tolerances and the effects of variable environmental conditions.